A chair includes a supporting structure, a seat moveably mounted on the supporting structure and moveable with respect to the supporting structure in a vertical direction, and a backrest including a first frame pivotally connected to the seat and a second frame separate from and pivotally mounted on the first frame to connect to the seat. Further, a first control device is mounted and concealed between the first and second frames in order to prevent the user from inadvertently colliding with it. The first control device is adjusted for various relative pivotal positions of the first and second frames.